Methods that employ inverters having a step-up device connected upstream are used in applications in which low direct voltages are converted into higher alternating voltages. The step-up device therein controls the output current of the direct current source and, as a direct current link voltage, delivers a direct voltage that is converted by the inverter into an alternating voltage and delivered to a load or alternating current system. The direct current link voltage is therein controlled by the inverter.
Methods of said type are used for, for example, connecting photovoltaic cells, fuel cells, and similar direct current sources having a varying specified draw current to a load or alternating current system. What is therein to be understood as a specified draw current is the draw current specified by, for instance, an internal controller of the direct current source for tapping the energy made available by the direct current source. Direct current sources of said type generally have a current-dependent characteristic voltage curve. External influencing factors, for example changing light conditions in the case of photovoltaic cells, can cause the draw current to vary. Dynamic operating conditions of said kind need to be taken onto account in the way the inverter and step-up device are controlled. According to the prior art there are various typologies therefor, examples of which can be found in U.S. 2004/0207366 and U.S. 2004/0165408. The direct current link voltage is therein maintained at a virtually constant value. The maximum permissible output voltage of the direct current source is decisive in determining said value because the maximum pulse duty factor of the step-up device is not allowed to be reached up to and including said value. The step-up device's current controller will otherwise no longer be able to control the draw current, or the current will be decreased owing to the voltage.
Specifying the constant direct current link voltage has disadvantages for the overall efficiency of the unit consisting of step-up device and inverter. The step-up device works most efficiently when operating with the maximum pulse duty factor. However, the pulse duty factor is significantly below the maximum when the direct current source is in its stationary operating condition because sufficient room still has to be provided for dynamic increases in draw current.